Rickettsia rickettsii and R. conorii are Gram-negative, obligate intracellular a-proteobacteria known to cause Rocky Mountain spotted fever (RMSF) and Mediterranean SF in humans. A unique trait of pathogenic rickettsiae is the tropism for microvascular endothelium of the blood vessels, which results in disseminated endothelial infection, vascular inflammation, and compromised vascular permeability. Activation of otherwise quiescent endothelium in response to rickettsial invasion is characterized by acquisition of prothrombotic, procoagulant and proinflammatory phenotypes, manifesting as 'rickettsial vasculitis'. Although endothelial activation typically involves input from multiple upstream mechanisms, including Janus Kinase-Signal Transducer and Activator of Transcription (JAK-STAT) proteins, the potential contributions of this critically important signaling pathway, which is typically involved in antiviral host defense, in rickettsial interactios with the host cell are completely unknown. Published as well as preliminary in vitro and in vivo evidence from our recent findings suggests: i) early STAT3 and late STAT1 activation in human microvascular endothelial cells (ECs) infected with SF rickettsiae; ii) ability of 'conditioned medium' from infected cells to activate STAT1 in nave ECs through IFN-b; iii) increased expression of Interferon-Stimulated Gene encoding protein of 15 kDa (ISG15) dependent on IFN-b and STAT1; and iv) anti-bacterial activities of IFN-b, STAT1 and ISG15 against rickettsiae. These novel findings are the basis of three independent albeit mechanistically overlapping specific aims. First, we will identify receptor-mediated and intracytoplasmic signaling mechanisms and consequences of JAK-STAT activation during infection of cultured human and murine ECs with spotted fever rickettsiae. Because ISG15 not only plays an important role in innate immunity, but can also bind to other cellular proteins and modulate their functions via ISGylation, the second aim will determine its role in host defense mechanisms against pathogenic rickettsiae. Specifically, we will identify and analyze the functions of ISGylated proteins in infected endothelium and determine whether cytokine-like activity of secreted ISG15 activates Natural Killer cell-mediated host defense. Finally, we will define the potential roles of IFN-b, STAT1, and ISG15 as critical determinants of host defense exploiting established as well as novel murine models of infection mimicking human disease and mice lacking IFNAR, STAT1, and ISG15 as the host. We will employ contemporary, cutting edge methods of cell biology, microscopy, molecular genetics, and proteomics to reveal unique insights into mechanisms underlying host defense vis--vis pathogenesis. Since rickettsioses are now re-emerging globally, comprehensive understanding of the pathogen interactions with the target host cell will aid in the identification of innovative therapeutic strategies to alter te course of debilitating rickettsial infections in favor of the host.